PDCD1 and cancer: Because treatment of canine cancers with anti-PD-L1 or anti-PD-1 antibodies is a fairly unexplored area, there were some limitations in these initial studies such as sample size as well as variation in study cases such as cancer type and stage, and status of prior treatment (i.e., treatment-naïve vs. history of prior treatment), which could have contributed to the inconsistency and variation in outcomes.